Improve parsing error message

[pya]

Currently a parsing error emits a not very user-friendly error message:

...
E   TypeError: 'NoneType' object is not iterable
------------------------------- Captured stdout --------------------------------
ParsingError: lineno=23 colno=7

The not useful part does not need to be displayed. Furthermore, some more details about the problem such as missing opening and closing parenthesis should be stated. Also, a message about a syntax error seems more sensible.